Patent application title: OPTICAL GLASS, PRECISION PRESS-MOLDING PREFORM, OPTICAL ELEMENT AND THEIR PRODUCTION PROCESSES AS WELL AS IMAGE-SENSING DEVICE

Abstract:

An optical glass that is an oxide glass and comprises, by cationic %, 20
to 40% of a total of Si4+ and B3+, 15 to 40% of a total of
Nb5+, Ti4+, W6+ and Zr4+, 0.2 to 20% of a total of
Zn2+, Ba2+, Sr2+ and Ca2+, and 15 to 55% of a total
of Li+, Na+ and K+, the cationic ratio of the content of
B3+ to the total content of B3+ and Si4+ being 0.01-0.5,
the cationic ratio of the content of Zr4+ to the total content of
Nb5+, Ti4+, W6+ and Zr4+ being 0.05 or less, the
molar ratio of the total content of Zn2+ and Ba2+ to the total
content of Zn2+, Ba2+, Sr2+ and Ca2+ being 0.8-1, the
optical glass having a refractive index nd of 1.815 or more and an Abbe's
number νd of 29 or less.

Claims:

1. An optical glass that is an oxide glass and comprises, by cationic %,20
to 40% of a total of Si4+ and B3+,15 to 40% of a total of
Nb5+, Ti4+, W6+ and Zr4+,0.2 to 20% of a total of
Zn2+, Ba2+, Sr2+ and Ca2+, and15 to 55% of a total of
Li+, Na+ and K+,the cationic ratio of the content of
B3+ to the total content of B3+ and Si4+ being
0.01-0.5,the cationic ratio of the content of Zr4+ to the total
content of Nb5+, Ti4+, W6+ and Zr4+ being 0.05 or
less,the molar ratio of the total content of Zn2+ and Ba2+ to
the total content of Zn2+, Ba2+, Sr2+ and Ca2+ being
0.8-1,the optical glass having a refractive index nd of 1.815 or more and
an Abbe's number νd of 29 or less.

2. The optical glass of claim 1, which has a glass transition temperature
of less than 530.degree. C.

3. The optical glass of claim 1 or 2, which has a liquidus temperature of
1,080.degree. C. or lower.

4. The optical glass of any one of claims 1 to 3, wherein the cationic
ratio of the content of Nb5+ to the total content of Nb5+ and
Ti4+, (Nb5+/(Nb5++Ti4+)), is 0.65-1.

5. The optical glass of any one of claims 1 to 4, which has an Si4+
content of 15 to 30%.

6. The optical glass of any one of claims 1 to 5, which has a B3+
content of 15% or less.

7. The optical glass of any one of claims 1 to 6, which has an Nb5+
content of 10 to 30%.

8. The optical glass of any one of claims 1 to 7, which has a Ti4+
content of 0 to 15%.

9. The optical glass of any one of claims 1 to 8, which has a W6+
content of 0 to 4%.

10. The optical glass of any one of claims 1 to 9, which has a Zr4+
content of 0 to 4%.

11. The optical glass of any one of claims 1 to 10, which has a Zn2+
content of 9% or less.

12. The optical glass of any one of claims 1 to 11, which has a Ba2+
content of 6% or less.

13. The optical glass of any one of claims 1 to 12, which has an Sr2+
content of 2% or less.

14. The optical glass of any one of claims 1 to 13, which has a Ca2+
content of 3% or less.

15. The optical glass of any one of claims 1 to 14, which has an Li+
content of 25% or less.

16. The optical glass of any one of claims 1 to 15, which has an Na+
content of 30% or less.

17. The optical glass of any one of claims 1 to 16, which has a K+
content of 25% or less.

18. The optical glass of any one of claims 1 to 17, wherein the cationic
ratio of the content of Li+ to the total content of Li+,
Na+ and K+ is 0.1-1.

19. The optical glass of any one of claims 1 to 18, which has a
ΔPg,F of 0.0130 or less.

20. A precision press-molding preform formed of the optical glass recited
in any one of claims 1 to 19.

21. A process for producing a precision press-molding preform, which
comprises manufacturing the preform recited in claim 20 through the steps
of heating and melting glass raw materials to prepare a molten glass and
shaping said molten glass.

22. An optical element formed of the optical glass recited in any one of
claims 1 to 19.

23. A process for producing an optical element, which comprises the steps
of heating the precision press-molding preform recited in claim 20 and
precision press-molding the preform with a press mold.

24. The process for producing an optical element as recited in claim 23,
wherein the precision press-molding preform and the press mold are heated
together and the preform is precision press-molded.

25. The process for producing an optical element as recited in claim 23,
wherein the precision press-molding preform is heated and then introduced
into the press mold that is pre-heated, and the preform is precision
press-molded.

26. An image-sensing device having the optical element recited in claim
22.

Description:

TECHNICAL FIELD

[0001]This invention relates to an optical glass having high-refractivity
high-dispersion properties and having excellent precision
press-moldability, a precision press-molding preform and an optical
element formed of the above glass each, and processes for producing them,
and it further relates to an image-sensing device having the above
optical element mounted thereon.

BACKGROUND ART

[0002]A high-refractivity high-dispersion optical glass is highly
demanded, and in particular, aspherical lenses formed of the above glass
are indispensable as lenses for high-performance digital still cameras.

[0003]As a method for mass-producing aspherical lenses, there is known a
precision press-molding method (which is also called "optical press
molding method"). As a high-refractivity high-dispersion optical glass
that is moldable by a precision press-molding method, a phosphate glass
is known. While the phosphate optical glass is an excellent glass, it
involves a problem that a glass surface is liable to be damaged during
its precision press-molding.

[0004]As a non-phosphate glass having high-refractivity high-dispersion
properties, glasses disclosed in Patent Documents 1 to 5 are known. All
of these glasses have silica-containing compositions.

[0008]For obtaining a high-refractivity high-dispersion glass, it is
required to introduce components that impart high-refractivity
high-dispersion, such as Nb, Ti, etc., regardless of whether it is a
phosphate-containing glass or silica-containing glass.

[0009]However, when a glass containing Nb, Ti, etc., is precision
press-molded, a redox reaction takes place in an interface between the
glass and a press mold. As a result, foaming takes place on the lens
surface, and there is hence caused a problem that it is not easy to
maintain production yields at high levels. The above problem becomes
conspicuous when the press-molding temperature increases. A
silica-containing glass has high glass transition temperature as compared
with a phosphate glass, and it is required to set a press-molding
temperature at high temperatures, which promotes the reaction between the
glass and a press mold during precision press-molding.

[0010]For example, the glass disclosed in Patent Document 1 has a glass
transition temperature of 530° C. or higher, and it is a glass
transition temperature insufficient for inhibiting the above reaction.
Further, the glass has low stability, and a crystal may be deposited
while a glass melt is stirred for obtaining a homogeneous optical glass,
or a crystal may be deposited when a glass melt is cast and molded, so
that the above glass is unsuitable for mass-production.

[0011]The glass disclosed in Patent Document 2 has a problem that it has
low stability like the glass in Patent Document 1 and is liable to be
devitrified.

[0012]Patent Document 3 discloses a high-refractivity high-dispersion
glass and an intermediate-refractivity high-dispersion glass. With regard
to the high-refractivity high-dispersion glass, however, the glass
transition temperature is not sufficiently decreased.

[0013]Under the circumstances, it is demanded to materialize a
high-refractivity high-dispersion optical glass that resists
devitrification and that enables the stable production of high-quality
optical elements by precision press-molding.

[0014]It is an object of this invention to overcome the above problems and
provide a high-refractivity high-dispersion optical glass excellent in
devitrification resistance and precision press-moldability, a precision
press-molding preform and an optical element formed of the above optical
glass each, processes for producing them, and an image-sensing device
having the above optical element.

Means to Solve the Problems

[0015]Means for solving the problems in this invention include:

[0016][1] an optical glass that is an oxide glass and comprises, by
cationic %,

[0017]20 to 40% of a total of Si4+ and B3+,

[0018]15 to 40% of a total of Nb5+, Ti4+, W6+ and
Zr4+,

[0019]0.2 to 20% of a total of Zn2+, Ba2+, Sr2+ and
Ca2+, and

[0020]15 to 55% of a total of Li+, Na+ and K+,

[0021]the cationic ratio of the content of B3+ to the total content
of B3+ and Si4+ being 0.01-0.5,

[0022]the cationic ratio of the content of Zr4+ to the total content
of Nb5+, Ti4+, W6+ and Zr4+ being 0.05 or less,

[0023]the molar ratio of the total content of Zn2+ and Ba2+ to
the total content of Zn2+, Ba2+, Sr2+ and Ca2+ being
0.8-1,

[0024]the optical glass having a refractive index nd of 1.815 or more and
an Abbe's number νd of 29 or less,

[0025][2] the optical glass of the above [1], which has a glass transition
temperature of less than 530° C.,

[0026][3] the optical glass of the above [1] or [2], which has a liquidus
temperature of 1,080° C. or lower,

[0027][4] the optical glass of any one of the above [1] to [3], wherein
the cationic ratio of the content of Nb5+ to the total content of
Nb5+ and Ti4+, (Nb5+/(Nb5++Ti4+)), is 0.65-1,

[0028][5] the optical glass of any one of the above [1] to [4], which has
an Si4+ content of 15 to 30%,

[0029][6] the optical glass of any one of the above [1] to [5], which has
a B3+ content of 15% or less,

[0030][7] the optical glass of any one of the above [1] to [6], which has
an Nb5+ content of 10 to 30%,

[0031][8] the optical glass of any one of the above [1] to [7], which has
a Ti4+ content of 0 to 15%,

[0032][9] the optical glass of any one of the above [1] to [8], which has
a W6+ content of 0 to 4%,

[0033][10] the optical glass of any one of the above [1] to [9], which has
a Zr4+ content of 0 to 4%,

[0034][11] the optical glass of any one of the above [1] to [10], which
has a Zn2+ content of 9% or less,

[0035][12] the optical glass of any one of the above [1] to [11], which
has a Ba2+ content of 6% or less,

[0036][13] the optical glass of any one of the above [1] to [12], which
has an Sr2+ content of 2% or less,

[0037][14] the optical glass of any one of the above [1] to [13], which
has a Ca2+ content of 3% or less,

[0038][15] the optical glass of any one of the above [1] to [14], which
has an Li+ content of 25% or less,

[0039][16] the optical glass of any one of the above [1] to [15], which
has an Na+ content of 30% or less,

[0040][17] the optical glass of any one of the above [1] to [16], which
has a K+ content of 25% or less,

[0041][18] the optical glass of any one of the above [1] to [17], wherein
the cationic ratio of the content of Li+ to the total content of
Li+, Na+ and K+ is 0.1-1.

[0042][19] the optical glass of any one of the above [1] to [18], which
has a ΔPg,F of 0.0130 or less,

[0043][20] a precision press-molding preform formed of the optical glass
recited in any one of the above [1] to [19],

[0044][21] a process for producing a precision press-molding preform,
which comprises manufacturing the preform recited in the above [21]
through the steps of heating and melting glass raw materials to prepare a
molten glass and shaping said molten glass,

[0045][22] an optical element formed of the optical glass recited in any
one of the above [1] to [19],

[0046][23] a process for producing an optical element, which comprises the
steps of heating the precision press-molding preform recited in the above
[20] and precision press-molding the preform with a press mold,

[0047][24] a process for producing an optical element as recited in the
above [23], wherein the precision press-molding preform and the press
mold are heated together and the preform is precision press-molded,

[0048][25] a process for producing an optical element as recited in the
above [23], wherein the precision press-molding preform is heated and
then introduced into the press mold that is pre-heated, and the preform
is precision press-molded, and

[0049][26] an image-sensing device having the optical element recited in
the above [22].

Effect of the Invention

[0050]According to this invention, there can be provided a
high-refractivity high-dispersion optical glass excellent in
devitrification resistance and precision press-moldability, a precision
press-molding preform and an optical glass formed of the above optical
glass each, processes for producing them, and an image-sensing device
having the above optical element.

BRIEF EXPLANATION OF THE DRAWINGS

[0051]FIG. 1 is a photograph of a lens obtained in Comparative Example 2.

EMBODIMENTS FOR PRACTICING THE INVENTION

[Optical Glass]

[0052]This invention provides a high-refractivity high-dispersion optical
glass which employs a silica-containing composition thereby to prevent
damage on a glass surface during precision press-molding, which maintains
high refractivity and at the same time renders a glass transition
temperature lower in order to prevent quality degradation caused on an
optical element surface by an interfacial reaction between a component
that imparts high-refractivity high-dispersion and the molding surface of
a press mold, the quality degradation being a problem inherent to the
precision press-molding of a high-refractivity high-dispersion optical
glass, and which hence enables the stable production of high-quality
optical elements by precision press-molding. Further, this invention also
provides an optical glass that has excellent glass stability and can be
produced easily in spite of its being a high-refractivity glass.

[0053]Further, there can be provided an optical glass material that
maintains high-refractivity high-dispersion properties and at the same
time has a partial dispersion ratio Pg,F controlled so that it is small,
that is imparted with a property remarkably close to a normal line in a
partial dispersion ratio Pg,F-Abbe's number νd chart and that is hence
very effective for correcting chromatic aberration when combined with a
lens formed of a low-dispersion glass. A silica-containing composition is
also preferred for materializing the above partial dispersion property.

[0054]The optical glass of this invention completed on the basis of the
above concept is an oxide glass and comprises, by cationic %,

[0055]20 to 40% of a total of Si4+ and B3+,

[0056]15 to 40% of a total of Nb5+, Ti4+, W6+ and
Zr4+,

[0057]0.2 to 20% of a total of Zn2+, Ba2+, Sr2+ and
Ca2+, and

[0058]15 to 55% of a total of Li+, Na+ and K+,

[0059]the cationic ratio of the content of B3+ to the total content
of B3+ and Si4+ being 0.01-0.5,

[0060]the cationic ratio of the content of Zr4+ to the total content
of Nb5+, Ti4+, W6+ and Zr4+ being 0.05 or less,

[0061]the molar ratio of the total content of Zn2+ and Ba2+ to
the total content of Zn2+, Ba2+, Sr2+ and Ca2+ being
0.8-1,

[0062]the optical glass having a refractive index nd of 1.815 or more and
an Abbe's number νd of 29 or less.

[0063]The optical glass of this invention will be explained hereinafter,
while contents and total contents of cationic components by % hereinafter
stand for contents or total contents by cationic % unless otherwise
specified.

[0064]Si4+ and B3+ are glass network-forming oxides and are
essential for maintaining glass stability and moldability of a molten
glass. When these components are incorporated to excess, however, the
refractivity decreases. The total content of Si4+ and B3+ is
hence limited to 20 to 40%. The total content of Si4+ and B3+
is preferably in the range of 25 to 35%, more preferably in the range of
25 to 34%, still more preferably 29 to 33%, yet more preferably in the
range of 30 to 33%.

[0065]In addition to the above effects, Si4+ not only has the effect
of inhibiting phase separation during precision press-molding but also
works to improve chemical durability and to inhibit a decrease in
viscosity during the shaping of a molten glass to maintain the molten
glass in a state suitable for shaping. When it is introduced to excess,
however, the glass transition temperature and liquidus temperature
increase, and the meltability and devitrification resistance decrease.
When the above phase separation is inhibited, a decrease in
transmissivity by whitening of the glass can be prevented.

[0066]In addition to the above effects, B3+ not only improves
meltability but also works to decrease glass transition temperature. When
it is introduced to excess, chemical durability is decreased. Since
meltability is improved, a homogeneous glass can be obtained without
setting the glass-melting temperature at a high temperature. As a result,
the corrosion of a crucible can be suppressed, and the coloring of a
glass caused by the melting of a material such as platinum in the glass
can be suppressed.

[0067]The above effects of Si4+ and B3+ are taken into account,
and the cationic ratio (B3+/(B3++Si4+)) of the content of
B3+ to the total content of B3+ and Si4+ is adjusted to
0.01-0.5. When the cationic ratio (B3+/(B3++Si4+)) is
adjusted to 0.01 or more, the glass transition temperature can be further
decreased, so that there can be improved the effect of inhibiting an
interfacial reaction between a glass and a press mold during precision
press-molding, and meltability and devitrification resistance can be
improved. When the cationic ratio (B3+/(B3++Si4+)) is
larger than 0.5, the viscosity when a molten glass is shaped decreases,
so that the molten glass is deteriorated in shapeability, and tendency to
phase separation during precision press-molding is increased, and
chemical durability is decreased. The cationic ratio
(B3+/(B3++Si4+)) is preferably in the range of 0.05 to
0.5, more preferably in the range of 0.08 to 0.5, still more preferably
in the range of 0.1 to 0.5, yet more preferably in the range of 0.1 to
0.45, further more preferably in the range of 0.1 to 0.4.

[0068]The content of Si4+ is preferably in the range of 15 to 30%,
more preferably in the range of 19 to 26%, still more preferably in the
range of 19 to 25.5%. The content of B3+ is preferably in the range
of 15% or less, more preferably 0.3 to 15%, still more preferably in the
range of 0.5 to 15%, yet more preferably in the range of 1 to 15%,
further more preferably in the range of 3 to 15%, still further more
preferably in the range of 6 to 12.3%, yet further more preferably in the
range of 7 to 12%.

[0069]Any one of Nb5+, Ti4+, W6+ and Zr4+ is a
component that has a big effect on the achievement of high refractivity
and high dispersion. When the total content of these components is less
than 15%, it is difficult to achieve the specified refractivity, and when
it exceeds 40%, devitrification resistance decreases, and liquidus
temperature increases. The total content of Nb5+, Ti4+,
W6+ and Zr4+ is hence adjusted to 15 to 40%. The above total
content is preferably in the range of 25 to 35%, more preferably in the
range of 26 to 33%, still more preferably in the range of 28 to 31%, yet
more preferably in the range of 28 to 30%.

[0070]In addition to the above effects, Nb5+ works to improve
devitrification resistance and to decrease liquidus temperature. Further,
it works to bring the partial dispersion property close to a normal line,
i.e., to bring ΔPg,F close to zero. When it is incorporated to
excess, the devitrification resistance is decreased, and the liquidus
temperature is increased.

[0071]In addition to the above effects, Ti4+ improves the
devitrification resistance and works to improve the chemical durability.
When it is incorporated to excess, however, the tendency to phase
separation during precision press-molding is intensified.

[0072]In addition to the above effects, W6+ improves devitrification
resistance and works to inhibit an increase in liquidus temperature. When
it is incorporated to excess, however, the devitrification resistance is
degraded, and the liquidus temperature is increased. Further, the
tendency to coloring is also intensified.

[0073]In addition to the above effects, Zr4+ not only inhibits phase
separation during precision press-molding, but also to works to improve
chemical durability and devitrification resistance. When it is
incorporated to excess, however, devitrification resistance is decreased,
and liquidus temperature is increased. Since the incorporation of
Zr4+ to excess increases the glass transition temperature and
promotes an interfacial reaction between a glass and a press mold during
precision press-molding, the cationic ratio
(Zr4+/(Nb5++Ti4++W6++Zr4+)) of the content of
Zr4+ to the total content of Nb5+, Ti4+, W6+ and
Zr4+ is limited to 0.05 or less. For obtaining the above effects of
Zr4+, the cationic ratio
(Zr4+/(Nb5+Ti4++W6++Zr4+)) is preferably in the
range of 0.005 to 0.05, more preferably 0.008 to 0.03, still more
preferably 0.009 to 0.025.

[0074]Of Nb5+, Ti4+, W6+ and Zr4+, Nb5+ and
Ti4+ are components that do not deteriorate devitrification
resistance even when they are introduced in a large amount, so that the
total content of Nb5+ and Ti4+ is preferably in the range of 20
to 35%, more preferably in the range of 26 to 29.5%, still more
preferably in the range of 26 to 28.5%. For further improving
devitrification resistance, it is preferred to incorporate Nb5+ and
Ti4+ as essential components.

[0075]Further, since Nb5+ of Nb5+ and Ti4+ works greatly to
keep the partial dispersion ratio low and to bring the partial dispersion
property close to a normal line, the cationic ratio
(Nb5+/(Nb5++Ti4+)) of the content of Nb5+ to the
total content of Nb5+ and Ti4+ is preferably in the range of
0.65 to 1. Since, however, the devitrification resistance is improved
when Nb5+ and Ti4+ are made co-present as glass components, the
cationic ratio (Nb5+/(Nb5++Ti4+)) is preferably in the
range of 0.65 to 0.9, more preferably in the range of 0.7 to 0.8.

[0076]When the effects of each of Nb5+, Ti4+, W6+ and
Zr4+ are taken into account, the preferred range of the content of
each component is as follows.

[0077]The upper limit of the content of Nb5+ is preferably 30%, more
preferably 23%, still more preferably 22%, yet more preferably 21%, and
the lower limit of the content of Nb5+ is preferably 10%, more
preferably 16%, still more preferably 18%, yet more preferably 19%. The
above preferred upper limits and lower limits may be combined as
required. Specifically, the content of Nb5+ is, for example,
preferably in the range of 10 to 30%, more preferably in the range of 16
to 23%, still more preferably in the range of 18 to 22%, yet more
preferably in the range of 19 to 21%.

[0078]The upper limit of the content of Ti4+ is preferably 15%, more
preferably 12%, still more preferably 10%, yet more preferably 9.5%,
further more preferably 9%, still further more preferably 8.5%, yet
further more preferably 8.0%, and the lower limit of the content of
Ti4+ is preferably 1%, more preferably 2%, still more preferably 3%,
yet more preferably 4%, further more preferably 5%, still further more
preferably 5.5%. The above preferred upper limits and lower limits may be
combined as required. Specifically, the content of Ti4+ is, for
example, preferably in the range of 0 to 15%, more preferably in the
range of 0 to 12%, still more preferably 0 to 10%, yet more preferably in
the range of 1 to 10%, further more preferably in the range of 2 to 10%,
still further more preferably in the range of 3 to 9%, yet further more
preferably 4 to 9%, further far more preferably in the range of 5 to
8.5%, particularly preferably in the range of 5.5 to 8.0%.

[0079]The upper limit of the content of W6+ is preferably 4%, more
preferably 3%, still more preferably 2.5%, yet more preferably 2.0%,
further more preferably 1.5%, and the lower limit of the content of
W6+ is preferably 0.5%. The above preferred upper limits and lower
limit may be combined as required. Specifically, the content of W6+
is, for example, preferably in the range of 0 to 3%, more preferably in
the range of 0 to 2.5%, still more preferably in the range of 0.5 to
2.0%, yet more preferably in the range of 0.5 to 1.5%.

[0080]The upper limit of the content of Zr4+ is preferably 4%, more
preferably 3%, still more preferably 2%, yet more preferably 1.5%,
further more preferably 1.2%, still further more preferably 1%, yet
further more preferably 0.6%, and the lower limit of the content of
Zr4+ is preferably 0.2%, more preferably 0.5%. The above preferred
upper limits and lower limits may be combined as required. Specifically,
the content of Zr4+ is, for example, preferably in the range of 0 to
4%, more preferably in the range of 0 to 3%, still more preferably 0 to
2%, yet more preferably in the range of 0 to 1.5%, further more
preferably in the range of 0.1 to 1.5%, still further more preferably in
the range of 0.2 to 1.2%, yet further more preferably in the range of 0.2
to 1%, further far more preferably in the range of 0.4 to 0.6%.

[0081]Zn2+, Ba2+, Sr2+ and Ca2+ are useful for
adjusting optical constants, and they are components that improve
devitrification resistance, meltability and light transmissivity and that
enhance a clarification effect when they are added to glass raw materials
in the form of carbonates and nitrates. When the total content of
Zn2+, Ba2+, Sr2+ and Ca2+ is less than 0.5%, it is
difficult to produce the above effects, and when it exceeds 20%,
devitrification resistance is decreased, liquidus temperature is
increased and chemical durability is decreased. The total content of
Zn2+, Ba2+, Sr2+ and Ca2+ is therefore limited to 0.2
to 20%. The upper limit of the total content of Zn2+, Ba2+,
Sr2+ and Ca2+ is preferably 15%, more preferably 10%, still
more preferably 8.5%, and the lower limit of the total content of
Zn2+, Ba2+, Sr2+ and Ca2+ is preferably 0.3%, more
preferably 0.4%, still more preferably 0.5%, yet more preferably 1%,
further more preferably 3%, further far more preferably 5%, further still
far more preferably 6.5%, further yet far more preferably 7%. The above
preferred upper limits and lower limits may be combined as required.
Specifically, the total content of Zn2+, Ba2+, Sr2+ and
Ca2+ is preferably in the range of 1 to 20%, more preferably in the
range of 3 to 20%, still more preferably in the range of 3 to 15%, yet
more preferably in the range of 5 to 10%, further more preferably in the
range of 6.5 to 10%, further far more preferably in the range of 7 to
8.5%.

[0082]In addition to the above effects, Zn2+ is excellent in work to
decrease glass transition temperature, and also works to maintain
refractivity at high levels. When it is incorporated to excess, however,
devitrification resistance is decreased, liquidus temperature is
increased, and chemical durability tends to be decreased.

[0083]In addition to the above effects, Ba2+ increases refractivity
and works to inhibit phase separation during precision press-molding.
When it is incorporated to excess, however, devitrification resistance is
decreased, liquidus temperature is increased, and chemical durability
tends to be decreased.

[0084]In addition to the above effects, Sr2+ works to increase
refractivity although its effect is lower than that of Ba2+.
Further, it works to inhibit phase separation during precision
press-molding. When it is incorporated to excess, however,
devitrification resistance is decreased, liquidus temperature is
increased, and chemical durability tends to be decreased.

[0085]In addition to the above effects, Ca2+ works to inhibit phase
separation during precision press-molding. When it is incorporated to
excess, however, devitrification resistance is decreased, liquidus
temperature is increased, glass transition temperature is increased, and
chemical durability tends to be decreased.

[0086]Of Zn2+, Ba2+, Sr2+ and Ca2+, Zn2+ is a
component that maintains high refractivity and at the same time highly
effectively decreases glass transition temperature, and Ba2+ is a
component that greatly works to increase refractivity, so that the
cationic ratio
((Zn2++Ba2+)/(Zn2++Ba2++Sr2++Ca2+)) of the
total content of Zn2+ and Ba2+ to the total content of
Zn2+, Ba2+, Sr2+ and Ca2+ is adjusted to 0.8-1 for
satisfying both high refractivity and low glass transition temperature.

[0087]The cationic ratio
((Zn2++Ba2+)/(Zn2++Ba2++Sr2++Ca2+) is
preferably in the range of 0.82 to 1, more preferably in the range of
0.85 to 1, still more preferably in the range of 0.9 to 1.

[0088]When the effects of each of Zn2+, Ba2, Sr2+ and
Ca2+ are taken into account, the preferred range of the content of
each component is as follows.

[0089]The content of Zn2+ is preferably in the range of 9% or less,
more preferably in the range of 1 to 9%, still more preferably in the
range of 3 to 9%, yet more preferably in the range of 3 to 8%, further
more preferably in the range of 4.5 to 6.5%, still further more
preferably in the range of 5.6 to 6.0%, the content of Ba2+ is
preferably in the range of 6% or less, more preferably in the range of
0.5 to 6%, still more preferably in the range of 0.5 to 4%, yet more
preferably in the range of 0.8 to 3%, further more preferably in the
range of 1.0 to 2.0%, the content of Sr2+ is preferably in the range
of 0 to 2%, more preferably in the range of 0 to 1.5%, still more
preferably in the range of 0 to 1%, and the content of Ca2+ is
preferably in the range of 0 to 3%, more preferably in the range of 0 to
2%, still more preferably in the range of 0 to 1.5%, yet more preferably
in the range of 0.1 to 0.9%, further more preferably in the range of 0.32
to 0.45%.

[0090]Li+, Na+ and K+ are components that improve
meltability and have the effect of decreasing glass transition
temperature. When the total content of these components is less than 15%,
it is difficult to produce the above effect, and when it exceeds 55%,
glass stability is decreased, and liquidus temperature is increased. The
total content of Li+, Na+ and K+ is hence limited to 15 to
55%. The total content of Li+, Na+ and K+ is preferably in
the range of 20 to 50%, more preferably in the range of 25 to 40%, still
more preferably in the range of 28 to 40%, yet more preferably in the
range of 30 to 35%.

[0091]Li+ is a component that has the largest effect of decreasing
glass transition temperature with maintaining high refractivity among the
alkali metal components. When it is incorporated to excess, however,
glass stability is decreased, and liquidus temperature is increased.

[0092]In addition to the above effects, when Na+ and K+ are made
co-present with Li+, they work to increase glass stability further
due to a mixing alkali effect.

[0093]For satisfying both high refractivity and low glass transition
temperature, the cationic ratio (Li+/(Li++Na++K+)) of
the total content of Li+ to the total content of Li+, Na+
and K+ in this invention is preferably adjusted to 0.1-1. From the
above point of view, the cationic ratio
(Li+/(Li++Na++K+)) is preferably in the range of 0.2
to 1, more preferably in the range of 0.3 to 0.8, still more preferably
in the range of 0.4 to 0.5.

[0094]When the effects of each of Li+, Na+ and K+ are taken
into account, the preferred range of the content of each component is as
follows.

[0095]The content of Li+ is preferably in the range of 25% or less,
more preferably in the range of 10 to 20%, still more preferably in the
range of 13 to 17%, yet more preferably in the range of 14 to 16%, the
content of Na+ is preferably in the range of 30% or less, more
preferably in the range of 10 to 20%, still more preferably in the range
of 13 to 17%, yet more preferably in the range of 14 to 16%, and the
content of K+ is preferably in the range of 0 to 25%, more
preferably in the range of 0 to 20%, still more preferably in the range
of 0 to 15%, yet more preferably in the range of 0 to 9%, further more
preferably in the range of 0 to 5%, still further more preferably in the
range of 2 to 4%.

[0096]La3+, Gd3+, Y3+ and Yb3+ work to increase
refractivity and to improve chemical durability. When each is
incorporated in an amount of over 6%, liquidus temperature is increased,
and devitrification resistance is decreased. The content of each of
La3+, Gd3+, Y3+ and Yb3+ is limited to 0 to 6%. The
content of each of La3+, Gd3+, Y3+ and Yb3+ is
preferably in the range of 0 to 3%, more preferably in the range of 0 to
2%, still more preferably in the range of 0 to 1%, and yet more
preferably, none of La3+, Gd3+, Y3 and Yb3+ is
incorporated.

[0097]Ta5+ also works to increase refractivity and to improve
chemical durability. When it is introduced in an amount of over 3%,
however, liquidus temperature is increased, and devitrification
resistance is decreased. The content of Ta5+ is hence limited to 0
to 3%. The content of Ta5+ is preferably in the range of 0 to 2%,
more preferably in the range of 0 to 1%.

[0098]Ge4+ is a network-forming oxide and works to increase
refractivity. Since, however, it is a very expensive component, the
content of Ge4+ is limited to 0 to 2%, and it is preferably limited
to 0 to 1%. And, more preferably, no Ge4+ is introduced.

[0099]Bi3+ increases refractivity and also works to improve glass
stability. When it is introduced in an amount of over 2%, however, the
coloring of a glass is intensified, so that the content of Bi3+ is 0
to 2%, preferably 0 to 1%, still more preferably zero.

[0100]Al3+ works to improve glass stability and chemical durability
when introduced in a small amount. When it is introduced in an amount of
over 1%, liquidus temperature is increased, and devitrification
resistance is decreased. The content of Al3+ is 0 to 1%, preferably
0 to 0.5%, more preferably 0 to 0.2%, still more preferably zero.

[0101]The glass of this invention is not required to contain any component
of Ga3+, Lu3+ and Hf4+. Since Ga3+, Lu3+ and
Hf4+ are also expensive components, the content of each of
Ga3+, Lu3+ and Hf4+ is preferably limited to 0 to 1%, it
is more preferably limited to 0 to 0.5%, and it is still more preferably
limited to 0 to 0.1%. And, it is particularly preferred to introduce none
of Ga3+, Lu3+ and Hf4+.

[0102]When adverse effects on the environment are taken into account, it
is also preferred to introduce none of As, Pb, U, Th, Te and Cd.

[0103]For making the most of excellent light transmissivity of the glass,
none of coloring-causing substances such as Cu, Cr, V, Fe, Ni, Co, etc.,
is introduced. That is, it is preferred to use none of these substances
as any glass raw material when the glass is produced. For achieving the
object of this invention, the total content of Si4+, B3+,
Nb5+, Ti4+, W6+, Zr4+, Zn2+, Ba2+,
Sr2+, Ca2+, Li+, Na+ and K+ is preferably 95 to
100%, more preferably 98 to 100%, still more preferably 99 to 100%.

[0104]To the optical glass of this invention, there can be added
externally 0 to 2 mass % of Sb2O2 and 0 to 2 mass % of
SnO2. These additives work as a clarifier, and Sb2O2 can
also inhibit impurities such as Fe, etc., from coloring the glass. The
amount of each of Sb2O2 and SnO2 externally added is
preferably 0 to 1 mass %, more preferably 0 to 0.5 mass %.

[0105]The optical glass of this invention is an oxide glass, and O2-
accounts for 50% or more of anionic components. Besides this, a small
amount of F.sup.-, Cl.sup.-, I.sup.- and Br.sup.- may be introduced. The
content of O2- is preferably in the range of 50 to 100 anionic %,
more preferably in the range of 80 to 100 anionic %, still more
preferably 85 to 100 anionic %, yet more preferably 90 to 100 anionic %,
further more preferably 95 to 100 anionic %, still further more
preferably 98 to 100 anionic %, yet further more preferably 99 to 100
anionic %, particularly preferably 100 anionic %.

[Refractive Index, Abbe's Number]

[0106]The optical glass of this invention has a refractive index nd of
1.83 or more and an Abbe's number νd of 29 or less. When the
refractive index nd is in the above range, there can be obtained an
optical glass suitable as a material for an optical element that
constitutes a high-function and compact optical system. When a
high-refractivity glass is used, the curvature of a lens surface can be
moderated when a lens having constant condensing power is produced. As a
result, the shapeability of a lens during precision press-molding can be
also improved.

[0107]When the Abbe's number νd is in the above range, there can be
obtained an optical glass suitable as a material for a lens that permits
excellent correction of chromatic aberration by using the lens in
combination with a lens formed of a low-dispersion glass.

[0108]The refractive index nd is preferably in the range of 1.83 to 1.90,
more preferably in the range of 1.83 to 1.88, still more preferably in
the range of 1.84 to 1.855, and the Abbe's number νd is preferably in
the range of 23 to 29, more preferably in the range of 24 to 25.5, still
more preferably in the range of 24.5 to 25.25.

[0109]When the refractive index is increased to excess, or when the Abbe's
number νd is decreased to excess, the glass stability decreases, or
the glass transition temperature tends to increase.

[Glass Transition Temperature]

[0110]The glass transition temperature of the optical glass of this
invention is lower than 530° C., preferably 520° C. or
lower, more preferably 515° C. or lower, still more preferably
510° C. or lower. As the glass transition temperature decreases,
the press-molding temperature can be set at lower temperatures. The
proceeding speed of the interfacial reaction between a glass and a press
mold during precision press-molding is greatly influenced depending upon
whether the press-molding temperature is high or low. Therefore, when the
glass transition temperature can be decreased by only several ° C.
or tens ° C., the interfacial reaction can be remarkably
inhibited.

[0111]Generally, when the refractive index is increased, the glass
transition temperature tends to increase. According to this invention,
however, there can be obtained a glass having a low glass transition
temperature suitable for precision press-molding in spite of being a
high-refractivity glass.

[Liquidus Temperature]

[0112]The liquidus temperature of the optical glass of this invention is
1,080° C. or lower, preferably 1,060° C. or lower, more
preferably 1,020° C. or lower, still more preferably 1,015°
C. or lower.

[0113]When the above liquidus temperature is maintained at a low level,
the temperature for shaping a molten glass can be decreased. When the
temperature for the above shaping is maintained at a low level, the
volatilization of easily volatile components such as boric acid, an
alkali metal, etc., from a molten glass surface can be inhibited, and the
variance of optical properties and the occurrence of surface striae can
be inhibited.

[0114]Generally, when the refractivity is increased, the liquidus
temperature tends to increase. According to this invention, however,
there can be obtained a glass having a low liquidus temperature excellent
for mass-producibility in spite of being a high-refractivity glass.

[0115]Generally, it is said to be difficult to achieve higher
refractivity, a lower glass transition temperature and a lower liquidus
temperature at the same time, while these properties can be
simultaneously materialized according to this invention.

[Partial Dispersion Property]

[0116]For achromatization of high order in an image-sensing optical
system, a projector optical system, etc., it is effective to use a lens
formed of a low-dispersion glass and a lens formed of a high-dispersion
glass in combination. Since, however, many glasses on the lower
dispersion side have large partial dispersion ratios, it is more
effective for correcting chromatic aberration of high order to combine a
lens formed of a glass having a small partial dispersion ratio in
addition of a high dispersion property.

[0117]According to this invention, there is provided a glass that is a
high-refractivity high-dispersion glass, that has a small partial
dispersion ratio and that is hence suitable for correcting chromatic
aberration of high order.

[0118]A partial dispersion ratio Pg,F is represented by (ng-nF)/(nF-nc) in
which ng, nF and nc are refractive indexes to g ray, F ray and c ray.

[0119]In a partial dispersion ratio Pg,F-Abbe's number νd chart, when a
partial dispersion ratio on a normal line as a reference for a normal
partial dispersion glass is taken as Pg,F.sup.(0), Pg,F.sup.(0) is
represented by the following expression using an Abbe's number νd.

Pg,F.sup.(0)=0.6483-(0.0018×νd)

[0120]ΔPg,F is a deviation of the partial dispersion ratio Pg,F from
the above normal line, and is represented by the following expression.

[0121]In the optical glass of this invention, the deviation ΔPg,F of
the partial dispersion ratio Pg,F is 0.014 or less, preferably 0.013 or
less, more preferably 0.012 or less, and the optical glass of this
invention is a glass suitable for well correcting chromatic aberration of
high order. The partial dispersion ratio Pg,F of the optical glass of
this invention is 0.610 to 0.620, preferably 0.611 to 0.618.

[Production of Optical Glass]

[0122]The optical glass of this invention can be obtained by weighing and
formulating oxides, carbonates, sulfates, nitrates, hydroxides, etc., as
raw materials so as to obtain an intended glass composition, fully mixing
them to obtain a mixture batch, and heating, melting, defoaming and
stirring the batch in a melting vessel to prepare a homogenous
bubble-free molten glass and shaping it. Specifically, it can be produced
by a known melting method.

[Precision Press-Molding Preform]

[0123]The precision press-molding preform of this invention will be
explained below.

[0124]The precision press-molding preform of this invention is
characteristically formed of the above optical glass of this invention.

[0125]The above precision press-molding preform (to be referred to as
"preform" hereinafter) refers to a glass gob that is used for precision
press-molding and is a glass shaped material having a mass equivalent to
the mass of a precision press-molded product.

[0126]The preform will be explained in detail below.

[0127]The preform means a pre-shaped glass material that is heated and
used for precision press-molding. As is well known, the precision
press-molding is also called optics molding and is a method in which the
optical function surface of an optical element is formed by transferring
the form of the molding surface of a press mold. The optical function
surface refers to that surface of an optical element which refracts,
reflects or diffracts light to be controlled or makes the light enter or
go out, and a lens surface of a lens, etc., correspond to the optical
function surface.

[0128]For preventing a reaction and fusion between a glass and the molding
surface of a press mold during precision press-molding and at the same
time making the extension of the glass good along the molding surface, it
is preferred to coat the preform surface with a release film. The release
film includes

[0129]noble metals (platinum, platinum alloy),

[0130]oxides (oxides of Si, Al, Zr, La, Y, etc.),

[0131]nitrates (nitrates of B, Si, Al, etc.), and

[0132]a carbon-containing film.

[0133]The carbon-containing film is desirably a film composed mainly of
carbon (a film having a greater content of carbon than the total content
of other elements when the contents of elements in the film are shown by
atomic %). Specific examples thereof include a carbon film, a hydrocarbon
film, etc. As a method for forming a carbon-containing film, there can be
employed known methods such as a vacuum vapor deposition method, a
spurring method, an ion plating method, etc., that use carbon materials,
or a known method such as thermal decomposition using a material gas such
as a hydrocarbon. The other films can be formed by a vapor deposition
method, a sputtering method, an ion plating method, a sol-gel method,
etc.

[0134]The preform is produced through the steps of heating and melting
glass raw materials to prepare a molten glass and shaping the molten
glass.

[0135]The first embodiment of producing a preform is a method in which a
molten glass gob having a predetermined weight is separated from the
molten glass and cooled to form a preform having a weight equivalent to
the above molten glass gob. For example, glass raw materials are melted,
clarified and homogenized to prepare a homogeneous molten glass, and the
molten glass is caused to flow out of a temperature-controlled outflow
nozzle made of platinum or platinum alloy. When a preform of a small size
or a spherical preform is shaped, the molten glass is caused to drop from
the outflow nozzle in the form of a molten glass drop having a desired
mass, received with a preform shaping die and shaped into the preform.
Alternatively, a molten glass drop having a desired mass is similarly
caused to drop in liquid nitrogen from the outflow nozzle, and then
shaped into the preform. When a preform of a middle or large size is
produced, the molten glass flow is caused to flow down from the outflow
pipe, the forward end of the molten glass flow is received with a preform
shaping die, a narrow portion is formed between the molten glass flow
nozzle and the preform shaping die, the preform shaping die is moved
vertically downward to separate a molten glass flow at the narrow portion
owing to the surface tension of the molten glass, and a molten glass gob
having a desired mass is received in a receiving member and shaped into
the preform.

[0136]For producing a preform having a smooth surface free of flaws,
soiling, creases, alteration, etc., e.g., a preform having a free
surface, there may be employed a method in which a molten glass gob is
shaped into a preform while causing the molten glass gob to float on/over
a preform shaping die by applying gas pressure, or a method in which the
molten glass gob is placed in a medium prepared by cooling a substance
that is a gas at room temperature under atmospheric pressure such as
liquid nitrogen to shape it into a preform.

[0137]When a molten glass gob is shaped into a preform while causing it to
float, a gas (called a floating gas) is blown to the molten glass gob,
and an upward gas pressure is applied thereto. In this case, when the
viscosity of the molten glass mass is too low, floating gas enters the
glass to form bubbles that remains in the preform. However, when the
viscosity of the molten glass gob is adjusted to 3 to 60 dPas, the glass
gob can be caused to float without any floating gas included in the
glass.

[0138]The gas to be used for blowing the floating gas to the preform
includes air, N2 gas, O2 gas, Ar gas, He gas, steam, etc. The
gas pressure is not specially limited so long as the preform can be
caused to flow without contacting solids such as the surface of the
shaping die, etc.

[0139]Many precision press-molded products (e.g., optical elements)
produced through performs have an axis of rotational symmetry like a
lens, so that the form of the preform is also preferably a form having an
axis of rotational symmetry. Specifically, the above form is a sphere or
a form having one axis of rotational symmetry. The form having one axis
of rotational symmetry includes a form having a smooth frame free of a
corner or a dent in its cross section including the above axis of
rotational symmetry, such as a form having a frame formed of an ellipse
of which the minor axis corresponds to the axis of rotational symmetry in
the above cross section, a form obtained by flattening a sphere (a form
obtained by selecting one axis passing the center of a sphere and pushing
down the sphere in the direction of the selected axis), a form in which
two points of intersection of a surface and the axis of symmetry exist, a
surface including one point of intersection is a concave surface and a
surface including the other point of intersection is a convex surface, a
form in which both the surfaces including the above two points of
intersection are concave surfaces, etc.

[0140]The second embodiment of producing a preform is a method in which a
homogeneous molten glass is cast into a casting mold to be shaped, strain
is removed from the shaped material by annealing, the shaped material is
cut or split to divide it into pieces having predetermined dimensions and
form to prepare a plurality of glass pieces, and the glass pieces are
polished to smooth surfaces and to obtain performs of a glass having a
predetermined mass each. The thus-produced preform is preferably
surface-coated with a carbon-containing film before it is used.

[Optical Element]

[0141]The optical element of this invention will be explained below. The
optical element of this invention is characteristically formed of the
above optical glass of this invention. Specific examples thereof include
lenses such as an aspherical lens, a spherical lens, a plano-concave
lens, a plano-convex lens, a biconcave lens, a biconvex lens, a convex
meniscus lens, a concave meniscus lens, etc., a micro lens, a lens array,
a lens with a diffraction grating, a prism, a prism with a lens function,
etc. The optical element may be provided with an anti-reflection film, a
partial anti-reflection film capable of wavelength selection, etc., on
its surface as required.

[0142]The optical element of this invention is formed of the glass having
a high-dispersion property but having a small partial dispersion ratio,
so that it can perform chromatic aberration of high order by combining it
with an optical element formed of other glass. Further, the optical
element of this invention is formed of the glass having high
refractivity, so that it can serve to downsize an optical system when
used in an image-sensing optical system, a projector optical system, etc.

[Process for Producing Optical Element]

[0143]The process for producing an optical element, provided by this
invention, will be explained below.

[0144]The process for producing an optical element, provided by this
invention, comprises the steps of heating the above precision
press-molding preform of this invention and precision press-molding it
with a press mold.

[0145]The steps of heating a press mold and a preform and pressing the
preform are preferably carried out in a non-oxidizing gas atmosphere of
nitrogen gas or a gas mixture of nitrogen gas with hydrogen gas in order
to prevent the oxidation of the molding surface of the press mold or a
release film formed on the above molding surface. In a non-oxidizing gas
atmosphere, a carbon-containing film coated on the preform surface is not
oxidized, and the above film remains on the surface of a molded product
obtained by precision press-molding. The above film is to be finally
removed, and for relatively easily and completely removing the
carbon-containing film, the precision press-molded product can be heated
in an oxidizing atmosphere, e.g., in air. The carbon-containing film is
to be oxidation-removed at a temperature at which the precision
press-molded product is not deformed by heating. Specifically, the
oxidation removal is preferably carried out in a temperature range of
lower than the transition temperature of the glass.

[0146]The precision press-molding uses a press mold having a molding
surface that is highly accurately processed beforehand in a desired form,
and a release film may be formed on the molding surface for preventing
the fusion of a glass during pressing. The release film includes a
carbon-containing film, a nitride film and a noble metal film, and the
carbon-containing film preferably includes a hydrogenated carbon film, a
carbon film, etc. In the precision press-molding, a preform is fed into
between a pair of an upper mold member and a lower mold member of which
the molding surfaces are accurately form-processed, both the mold and the
preform are heated up to a temperature corresponding to a glass viscosity
of 105 to 109 dPas, to soften the preform, and the preform is
press-molded to transfer the forms of the molding surfaces of the mold to
the glass.

[0147]Further, a preform that is temperature-increased to a temperature
corresponding to a glass viscosity of 104 to 108 dPas is fed
into between a pair of an upper mold member and a lower mold member of
which the molding surfaces are accurately form-processed, and the preform
is press-molded, whereby the forms of the molding surfaces of a mold can
be accurately transferred to a glass.

[0148]The pressure and time period for the pressing can be determined as
required by taking account of the viscosity of a glass, etc. For example,
the pressure for the pressing can be set at approximately 5 to 15 MPa,
and the pressing time period can be 10 to 300 seconds. The pressing
conditions such as the pressing time period, the pressing pressure, etc.,
can be determined depending upon the form and dimensions of a molded
product as required so far as they are known.

[0149]Then, the mold and the precision press-molded product are cooled,
and preferably, when a temperature of a strain point or lower is reached,
the precision press-molded product is separated from the mold and taken
out. In addition, for accurately bringing the optical properties into
agreement with desired values, annealing conditions of the molded product
during cooling, e.g., an annealing speed, etc., may be adjusted as
required.

[0150]The above process for producing an optical element is largely
classified into two methods. The first method is a method of producing an
optical element in which a preform is introduced into a press mold and
the press mold and the glass material (preform) are heated together, and
this method is recommendable when a high priority is put on improvements
of molding accuracy such as surface accuracy, decentering accuracy, etc.
The second method is a method of producing an optical element in which a
preform is heated and introduced into a pre-heated press mold to carry
out precision press-molding, and this method is recommendable when a high
priority is placed on the improvement of productivity.

[0151]The optical element of this invention can be also produced without a
press-molding step. For example, it can be obtained by casting a
homogeneous molten glass into a casting mold to form a glass block,
annealing the glass block to remove a strain, concurrently adjusting
optical properties by adjusting annealing conditions so as to obtain a
desired value of refractivity of the glass, then, cutting or splitting
the glass block to prepare a glass piece and grinding and polishing the
glass piece to complete it into the optical element.

Examples

[0152]This invention will be explained further in detail with reference to
Examples, while this invention shall not be limited by these Examples.

Example 1

[0153]Corresponding oxides, carbonates, sulfates, nitrates, hydroxides
were used as raw materials for introducing components that would give
glass compositions shown in Tables 1 to 7. The raw materials were weighed
and fully mixed to prepare a formulated raw material, and it was placed
in a platinum crucible, heated and melted. After melted, a molten glass
was cast into a casting mold, and it was gradually cooled to a
temperature around its glass transition temperature and was placed in an
annealing furnace immediately thereafter to carry out annealing treatment
in the range of transition temperature of the glass for about 1 hour.
Then, the glass was allowed to cool in the furnace until its temperature
reached room temperature. In this manner, optical glasses of glasses Nos.
1 to 47 were obtained.

[0154]In the thus-obtained optical glasses, no crystal observable through
a microscope was deposited.

[0155]Tables 1 to 7 show various properties of the above-obtained optical
glasses.

[0156]The optical glasses were measured for various properties by the
following methods.

[0157](1) Refractive Indexes nd, ng, nF and nc and Abbe's Number νd

[0158]A glass obtained by temperature decrease at a temperature decrease
rate of -30° C./hour was measured for refractive indexes, nd, ng,
nF, nc and an Abbe's number νd according to the refractivity measuring
method of Japan Optical Glass Industrial Society Standard.

[0159](2) Liquidus Temperature LT

[0160]A glass was placed in a furnace heated to a predetermined
temperature and held therein for 2 hours, and after it was cooled, the
glass was internally observed through an optical microscope of 100
magnifications. The liquidus temperature of the glass was determined on
the basis of whether or not a crystal was present.

[0161](3) Glass Transition Temperature Tg

[0162]A glass was measured at a temperature elevation rate of 10°
C./minute with a differential scanning calorimeter (DSC).

[0165](5) Deviation ΔPg,F of Partial Dispersion Ratio from Normal
Line

[0166]A deviation was calculated from a partial dispersion ratio
Pg,F.sup.(0) on a normal line calculated from a partial dispersion ratio
Pg,F and an Abbe's number νd.

Comparative Example 1

[0167]Glasses were melted according to a method described in Patent
Document 2 so as to obtain compositions of Examples 1 to 13 of Patent
Document 2. In compositions of Examples 1 and 2, glasses were devitrified
while they were stirred, and in those of Examples 4 to 13, no glasses
were formed. In a compositions of Example 3, a melt was cast into a
casting mold to give a glass, while the deposition of a crystal was
observed inside.

Example 2

[0168]Glass raw materials prepared so as to give the optical glasses
produced in Example 1 were melted, clarified and homogenized to obtain
molten glasses. Each molten glass was dropped from a nozzle made of
platinum and received with a preform shaping mold, and a spherical
preform was shaped while causing it to float by applying gas pressure. In
this manner, spherical performs formed of the above various glasses were
formed.

[0169]Further, the above molten glass was caused to continuously flow out
of a pipe formed of platinum, its lower end portion was received with a
preform, and after a narrow portion was formed in the molten glass flow,
the preform receiving mold was moved rapidly vertically downward to cut
the molten glass flow at the narrow portion. A separated glass gob was
received on the preform shaping mold, and a preform was formed while it
was caused to float by applying gas pressure. In this manner, preforms
formed of the above various glasses were formed.

[0170]The thus-obtained preforms were optically homogeneous and had high
quality.

Example 3

[0171]Molten glasses prepared in Example 2 were caused to continuously
flow out and cast into casting molds to form glass blocks, and the glass
blocks were annealed and cut to obtain a plurality of glass pieces each.
These glass pieces were ground and polished to obtain preforms formed of
the above various glasses.

[0172]The thus-obtained preforms were optically homogeneous and had high
quality.

Example 4

[0173]The preforms prepared in Examples 2 and 3 were surface-coated with a
carbon-containing film each. Each preform was separately introduced into
a press mold having upper and lower mold members and sleeve member made
of SiC, and such a preform and the press mold were heated together in a
nitrogen atmosphere to soften the preform, and the preform was precision
press-molded. In this manner, there were obtained various lenses formed
of the above various glasses, such as aspherical convex meniscus lenses,
aspherical concave meniscus lenses, aspherical biconvex lenses and
aspherical biconcave lenses. The conditions for the precision
press-molding were adjusted within ranges described already.

[0174]When the thus-obtained various lenses were observed, there were
found no white turbidity caused by phase separation, etc., and none of
damage, cloudiness and breaking were found on the lens surfaces.

[0175]The above process was repeated to carry out a mass-production test
with regard to the various lenses, while failures such as the fusion of a
glass and a press mold, etc., did not take place, and there could be
highly accurately produced lenses that had high quality on surfaces and
inside. The thus-produced lenses may be surface-coated with an
anti-reflection film each.

[0176]The above preforms coated with a carbon-containing film each were
heated to soften them, and each preform was separately introduced into a
pre-heated press mold and precision press-molded. In this manner, there
were produced various lenses of the above various lenses such as
aspherical convex lenses, aspherical concave meniscus lenses, aspherical
biconvex lenses and aspherical biconcave lenses. The conditions for the
precision press-molding were adjusted within ranges described already.

[0177]When the thus-obtained various lenses were observed, there were
found no white turbidity caused by phase separation, etc., and none of
damage, cloudiness and breaking were found on the lens surfaces.

[0178]The above process was repeated to carry out a mass-production test
with regard to the various lenses, while failures such as the fusion of a
glass and a press mold, etc., did not take place, and there could be
highly accurately produced lenses that had high quality on surfaces and
inside. The thus-produced lenses may be surface-coated with an
anti-reflection film each.

[0179]Various optical elements such as prisms, micro lenses, lens array,
etc., can be also produced by changing the molding surface of the press
mold as required.

Comparative Example 2

[0180]An optical glass shown in the following Table 8 was prepared, and
precision press-molding preforms were produced from the glass. When the
preforms were precision press-molded in the same manner as in Example 4
to produce lenses, and many bubbles were found in the lens surfaces.
FIGS. 1(a) and 1(b) show magnified photographs taken of the lens
surfaces.

[0181]Various interchangeable lenses for a single-lens reflex camera were
made from various lenses produced in Example 4.

[0182]Further, various optical systems for compact digital cameras were
made from various lenses produced in Example 4 and modularized. Further,
image sensors such as CCD, CMOS, etc., were attached to these optical
systems for modularization.

[0183]When various lenses produced in Example 4 are used as described
above, there can be obtained high-function compact optical systems,
interchangeable lenses, lens modules and image-sensing devices. When
lenses produced in Example 4 are combined with lenses formed of
high-refractivity low-dispersion optical glasses, there can be obtained
various optical systems that perform chromatic aberration of high order
and image-sensing devices having these optical systems.

Industrial Utility

[0184]The optical glass of this invention has high-refractivity
high-dispersion properties and excellent devitrification resistance and
has a low glass transition temperature, and it is an optical glass
excellent for precision press-molding. Further, it is an optical glass
suitable for correcting chromatic aberration of high order, and is
suitably used for precision press-molding preforms and optical elements.